The present invention is related to ceramic compositions containing SiC and alpha prime/beta prime sialon, and possessing a combination of high hardness and high toughness.
Prior art commercial SiC components such as seal rings, nozzles and bearings, while having high room temperature hardness, high abrasion resistance and high thermal conductivity, have poor fracture toughness, making them unsuitable for, or reducing their lifetime in, many applications where impact may occur.
The range of useful applications of SiC components is further limited by the fabrication methods which are used to fabricate dense SiC articles. Fabrication of commercial SiC articles is performed by (1) hot pressing, which is a method that is usually only economical for relatively simple final component shapes; (2) sintering which, while allowing more complex shapes to be made, is also very costly due to the high temperatures (2000.degree.-2300.degree. C.) required to densify SiC; and (3) reaction bonding which, while capable of producing complex shapes, has inferior properties due to the presence of free silicon.
Alpha prime-beta prime sialons are described in U.S. Pat. Nos. 4,563,433 and 4,547,470, and are exemplified by the commercial prior art composition, KYON 2000, which contains about 5 to 40 w/o (weight percent) alpha prime sialon, with the remainder being essentially beta prime sialon, except for minor phases which are mainly a result of sintering aid residues. (KYON is a trademark of Kennametal Inc. for its ceramic compositions and articles.) Alpha prime-beta prime sialons, while having higher fracture toughness, have lower hardness, abrasion resistance, thermal conductivity and a lower Young's modulus compared with commercial SiC.
Alpha prime-beta prime sialons, while having higher hardness than beta prime sialons, have lower fracture toughness than beta prime sialons with fracture toughness decreasing as alpha prime content increases. Beta prime sialons are described in U.S. Pat. Nos. 3,991,166 and 4,127,416, and are exemplified by the prior art commercial composition, KYON-3000, which is all beta prime sialon except for minor phases which are a result of sintering aid residues.
Beta prime sialon has the formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z, where O&lt;z.ltoreq.5. Alpha prime sialon has the formula (Si,Al).sub.12 M.sub.x (O,N).sub.16, where M is a sintering aid metal.
A ceramic composite composition of very fine (1 micron or less average size) SiC particles in a beta prime sialon ceramic matrix is referred to in Richerson U.S. Pat. No. 3,890,250.
There therefore continues to be a need for an improved ceramic composition having both a hardness approaching pure SiC while also having high toughness.